Field devices are often used in the industry to measure various process variables, such as pressure, temperature, flow or product level. In particular, a radar level gauge system may be used for measuring the level of a product such as a process fluid, a granular compound or another material.
In a radar level gauge system, an electromagnetic transmit signal may be propagated from a transceiver arranged at the top of a tank towards a surface of a product in the tank, and an electromagnetic reflection signal, resulting from reflection of the transmit signal at the surface, returns to the transceiver. Based on the transmit signal and the reflection signal, the distance to the surface of the product can be determined, from which the level of the product in the tank can be deduced.
In many facilities, such as process industrial facilities, where field devices are used, there is existing wiring between the field device and a remote location, such as a host. The existing wiring often forms a two-wire current loop for communication of commands from the host to the field device and a measurement signal indicative of a process variable from the field device to the host. Communication on the two-wire current loop may take place by controlling the current flowing through the loop. In a 4-20 mA loop, the measurement signal may, for example, be the loop current itself. As an alternative or complement to such an analog current level, digital information may be communicated by modulating the loop current, for example using the HART-protocol.
In addition to being able to receive and send signals over the two-wire current loop, the field device may need to be able to operate using electric power drawn from the two-wire current loop. In other words, the field device may be a loop-powered field device.
In some existing solutions for realizing a loop-powered field device, the measurement device for determining the process variable, a current control circuit for providing the measurement signal to the two-wire current loop, and a shunt regulator may all be connected in parallel. However, due to expected variations in the loop voltage, a relatively large portion of the available electrical power on the loop is not used for powering the measurement device in such a solution.
To improve the energy efficiency of a loop-powered field device, U.S. Pat. No. 8,477,064 discloses electrically connecting the current control circuit in series with a converter for converting input power from the current loop to output power to the measurement device, and providing a voltage regulator for keeping the voltage across the current control circuit constant by varying the voltage across the converter. In this way, practically all of the available power on the loop can be used for powering the measurement device, regardless of the loop voltage.
Although the loop-powered field device according to U.S. Pat. No. 8,477,064 provides for very energy efficient operation, there may be room for improvement, in particular concerning the stability of the regulated loop current.